1. Field of the Invention
The present invention relates to a semiconductor device used in high frequencies ranging from microwave band to millimeter wave band and, particularly, to a semiconductor device capable of transmitting signals to the semiconductor elements and circuit components without deteriorating the characteristics of high-frequency signals. The invention further relates to a mounted structure in which a high-frequency semiconductor device is mounted on a mother board minimizing the degradation of the characteristics of high-frequency signals.
2. Description of the Prior Art
There have heretofore been known high-frequency semiconductor devices for handling microwave and millimetric wave signals having structures as shown, for example, in FIGS. 18 and 19. In these semiconductor devices, a semiconductor element 73 for high-frequency use is mounted in a cavity 72 formed by a cap 71 and an insulating board 70 made of a dielectric material, the cavity 72 being air-tightly sealed. In the semiconductor device of FIG. 18, a signal transmission line 74 such as strip line is electrically connected to the semiconductor element 73 by bonding a wire or using a ribbon, drawn out of the cavity 72 passing through a wall member 75, and is extended up to the bottom surface of the semiconductor device passing over the side surface of the board 70 (Japanese Laid-Open Patent Publication No. 168939/1986). Referring to FIG. 19, furthermore, a signal transmission line 78 is formed on the bottom surface of the insulating board 70 and is connected to the semiconductor element 73 via a through-hole 79.
That is, in these semiconductor devices, high-frequency signals are usually input and output through a transmission line 74 or 78 arranged on the bottom surface of the insulating board 70. Besides, the semiconductor device is mounted on a mother board by connecting the transmission line 74 or 78 to a wiring layer 77 of the mother board 76 by soldering or the like method.
In the semiconductor device of FIG. 18, however, the width of the signal line must be narrowed at a portion where the transmission line 74 passes through the wall member 75. This is to change the signal line from a microstrip line into a strip line. As a result, reflection loss and radiation loss easily occur in this passage portion causing characteristics of the transmitted high-frequency signals to be deteriorated. Moreover, since the transmission line 74 is folded by the side surface of the substrate 70, high-frequency signals of millimeter wave band are mostly reflected by the folded transmission line 74 and are scarcely received. Besides, since a transmission line is formed on the side surface of the board mounting the semiconductor element, the semiconductor device itself inevitably becomes bulky making it difficult to realize a circuit board mounting the semiconductor device in a small size.
In the semiconductor device shown in FIG. 19, on the other hand, the transmission line 78 is connected to the semiconductor element 73 via the through-hole 79. Therefore, the transmission line neither passes through the wall member nor is folded, and characteristics of the signals are less deteriorated. As the frequency of the transmitted signals becomes higher than 10 GHz, however, the transmission loss in the through-hole 79 sharply increases, making it difficult to transmit the microwave to millimetric wave signals without deteriorating their characteristics.